feat(voyage): bump trekbrief-template to brief_version 2.1 + add phase_signals fixtures

plugins/voyage/tests/fixtures/brief-with-phase-signals.md

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+---
+type: trekbrief
+brief_version: "2.1"
+created: 2026-05-13
+task: "Add per-phase effort dialog to /trekbrief"
+slug: phase-signals-example
+project_dir: .claude/projects/2026-05-13-phase-signals-example/
+research_topics: 2
+research_status: complete
+auto_research: false
+interview_turns: 6
+source: interview
+phase_signals:
+  - phase: research
+    effort: low
+    model: sonnet
+  - phase: plan
+    effort: standard
+  - phase: execute
+    effort: high
+    model: opus
+  - phase: review
+    effort: standard
+---
+
+# Task: Phase-signals example
+
+## Intent
+
+A minimal brief that exercises the v5.1 phase_signals additive field with a
+mix of effort levels and model overrides. Used by tests/validators to confirm
+the validator accepts well-formed signals across the supported tier matrix.
+
+## Goal
+
+Validator returns valid: true. annotate.mjs strips phase_signals from the
+rendered HTML body (frontmatter stays in source).
+
+## Success Criteria
+
+- Validator passes.
+- annotate.mjs determinism: re-run produces byte-identical HTML.

plugins/voyage/tests/fixtures/brief-without-phase-signals.md

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+---
+type: trekbrief
+brief_version: "2.0"
+created: 2026-05-13
+task: "Backward-compat fixture for v5.0-style brief"
+slug: legacy-brief-example
+project_dir: .claude/projects/2026-05-13-legacy-brief-example/
+research_topics: 0
+research_status: complete
+auto_research: false
+interview_turns: 3
+source: interview
+---
+
+# Task: Legacy brief example
+
+## Intent
+
+A pre-v5.1 brief that pre-dates the phase_signals field. Used by
+tests/validators to confirm backward-compatibility: the brief is accepted
+without phase_signals as long as brief_version is < 2.1.
+
+## Goal
+
+Validator returns valid: true. The sequencing gate
+(BRIEF_V51_MISSING_SIGNALS) does NOT fire for brief_version 2.0.
+
+## Success Criteria
+
+- Validator passes.
+- No BRIEF_V51_MISSING_SIGNALS error in r.errors.